1. Field of the Invention
The present invention relates generally to a drill bit shank for rotary drill bits for drilling subterranean formations and to rotary drill bits so equipped.
2. State of the Art
A typical rotary drill bit includes a bit body secured to a hardened steel shank having a threaded pin connection for attaching the bit to a drill string, and a crown including a face region carrying cutting structures for cutting into an earth formation. Generally, if the bit is a fixed-cutter or so-called “drag” bit or drill bit, the cutting structures include a plurality of cutting elements formed at least in part of a superabrasive material, such as polycrystalline diamond. Rotary drag bits employing polycrystalline diamond compact (PDC) cutters have been employed for several decades. Typically, the bit body may be formed of steel, or a matrix of hard particulate material such as tungsten carbide (WC) infiltrated with a binder, generally of a copper alloy.
In the case of steel body drill bits, the bit body may typically be machined from round stock to a desired shape. Radially and longitudinally extending blades, internal watercourses for delivery of drilling fluid to the bit face, and topographical features defined at precise locations on the bit face may be machined into the bit body using a computer-controlled, multi-axis machine tool. Hard-facing for resisting abrasion during drilling is usually applied to the bit face and to other critical areas of the bit exterior, and cutting elements are secured to the blades on the bit face, generally by inserting the proximal ends thereof into cutting element pockets machined therein. After machining and hardfacing, the bit body may be secured to a hardened steel shank having a threaded pin connection for securing the steel body rotary drill bit to the drive shaft of a downhole motor or directly to drill collars at the distal end of a drill string rotated at the surface by a rotary table or top drive.
Matrix-type drill bits, on the other hand, include a bit body formed of a matrix of hard particulate material such as tungsten carbide contained within a graphite mold and infiltrated with a binder, generally of a copper alloy. Cast resin-coated sand, graphite displacements or, in some instances, tungsten carbide particles in a flexible polymeric binder, may be employed to define internal watercourses and passages for delivery of drilling fluid to the bit face, cutting element sockets or pockets, ridges, lands, nozzle apertures, junk slots and other external topographic features of the matrix-type rotary drag bit. However, because a matrix material comprising tungsten carbide or other relatively hard particles may be substantially unmachinable, a machinable steel blank is typically disposed within the bit mold prior to infiltration of the matrix material, the steel blank forming a portion of the matrix-type rotary drag bit body upon hardening of the infiltrant that affixes the blank therein. In a manner similar to fabrication of steel body drill bits, the matrix-type bit body, via the machinable blank, may be secured to a hardened steel shank having a threaded pin connection for securing the bit to the drive shaft of a downhole motor or directly to drill collars at the distal end of a drill string rotated at the surface by a rotary table or top drive.
Thus, in either steel body or matrix-type rotary drill bits, alignment between the bit body and the hardened shank is critical because the shank, which includes the threaded pin connection, may determine the axis of rotation of the bit. Alignment of the axis of rotation in relation to the cutting element design is obviously of great importance in the operation of a rotary drag bit because aspects of the rotary drill bit design may be based, at least in part, on cutting element positions as well as predicted forces thereon. For instance, so-called “anti-whirl” designs utilize a preferential lateral force directed toward a pad designed to ride against the formation in order to stabilize the rotary drag bit. Conventionally, a threaded connection has been employed between matrix-type bit bodies and the hardened shank, as described in more detail hereinbelow.
FIGS. 1A and 1B illustrate a conventional matrix-type drill bit 10 formed generally according to the description above. Conventional matrix-type drill bit 10 includes a central longitudinal axis 3 and bore 12 therethrough for communicating drilling fluid to the face of the bit during drilling operation. Cutting elements 5 and 7 (typically diamond, and most often a synthetic polycrystalline diamond compact or PDC) may be bonded to the bit face during infiltration of the bit body if thermally stable PDCs, commonly termed TSPs, are employed, or may be subsequently bonded thereto, as by brazing, adhesive bonding, or mechanical affixation.
The conventional preformed, so-called blank 14 comprising relatively ductile steel may also provide internal reinforcement of the bit body matrix 19. The blank 14 may be typically comprised of relatively ductile steel because the high temperatures experienced by the blank during infiltration may generally anneal most steel materials. Blank 14 may comprise a cylindrical or tubular shape, or may be fairly complex in configuration and include protrusions corresponding to blades, wings or other features on the bit face. The protrusions or fingers may be generally welded into longitudinal slots formed within the tubular portion of blank 14. The blank 14 and other preforms as mentioned above may be placed at appropriate locations within the graphite mold used to cast the bit body. The blank 14 may be affixed to the bit body matrix 19 upon cooling of the bit body after infiltration of the tungsten carbide with the binder in a furnace, and the other preforms are removed once the matrix has cooled. Blank 14 may be machined and affixed to shank 16 by way of threaded connection 15 as well as weld 20. Conventionally, a continuous weld may be formed between shank 16 and blank 14. The shank 16 typically is formed from an AISI 4140 steel, a material having a carbon equivalent of higher than about 0.35%, which requires the shank 16 and blank 14 to be preheated prior to welding. Shank 16 includes tapered threads 17 machined at the upper portion thereof for connecting the conventional matrix-type drill bit 10 to a string of drill pipe (not shown). Machined tapered threads 17 are formed prior to attachment of the shank 16 to the blank 14; therefore, proper alignment of the shank 16 with the blank 14 is critical.
FIG. 1C shows another conventional matrix-type drill bit 11 having a conventional shank 16 and illustrates the interface between the shank 16 and bit body 23. Conventional matrix-type drill bit 11 includes an internal bore 12 generally centered about the central longitudinal axis 3 thereof. Shank 16 includes tapered threads 17 for attachment to a drill string (not shown) as well as “bit breaker” surface 21 for loosening and tightening the tapered thread connection between the matrix drill bit 11 and the drill string (not shown). Shank 16 may be affixed to the bit body 23 by threaded connection 15 as well as weld 20. Of course, bit body 23 includes a blank (not shown) that provides the interfacing surface between the bit body 23 and the shank 16.
FIG. 1D shows a conventional steel body drill bit 30 including bit body 44 and internal bore 32 generally centered about central axis 33. As FIG. 1D shows, conventional steel body rotary drill bit 30 includes shank 36 comprising a bit breaker surface 41 and having a threaded connection 37 for connecting to a drill string wherein the shank 36 is affixed by weld 40 to the bit body 44. Bit body 44 may also carry blade(s) 42 having cutting elements 38 for removing formation during subterranean drilling.
As may be seen in FIGS. 1C and 1D, in manufacturing either a matrix-type or steel body rotary drill bit, a shank is affixed to a bit body. In addition, in conventional welding of a shank to a bit body of a rotary drill bit, the shank may comprise a material having a carbon equivalent of higher than about 0.35%, such as, for example, an AISI 4140 steel. Therefore, the shank and bit body may be preferably preheated to about 700° Fahrenheit before welding begins. Further, conventional welding procedures may designate that as the shank is welded to the bit body, if the temperature of the shank reaches 900° Fahrenheit the welding procedure may be interrupted until the temperature is reduced. When the conventional weld procedure resumes subsequent to delay caused by either overheating or inadequate heating of the shank, the weld may continue from substantially the same circumferential position as occurred at initiation of the delay.
U.S. Pat. No. 6,116,360 discloses, in discussing a prior art steel bodied bit, a shank welded to a steel bit body that protrudes therein. However, the mating surfaces between the shank and the steel bit body are not tapered.
In addition, U.S. Pat. No. 5,150,636 to Hill discloses a shrink-fit between a cutting head and a shank. Further, Hill discloses that the tip of the shank may have a slight reverse taper to better retain the cutting head.
It has been observed by the inventors herein that the conventional threaded connection between the shank and blank may generate undesirable stresses within the threaded joint and proximate weld joint. In addition, the conventional threaded connection may produce misalignment between the shank and bit body. Further, it has been observed that a conventional single-pass weld between the blank and shank may allow or even promote distortion and misalignment therebetween. Thus, it would be advantageous to eliminate the need for preheating of the shank prior to welding the shank to the bit body and a need exists for an improved shank configuration for use in fabricating rotary drill bits.